Oxide Ion Transport in Bismuth-Based Materials
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Oxide Ion Transport in Bismuth-Based Materials Rose-Noëlle Vannier, Edouard Capoen, Caroline Pirovano, César Steil, Guy Nowogrocki, Gaëtan Mairesse Laboratoire de Cristallochimie et Physicochimie du Solide, CNRS UMR 8012, ENSCL, Université des Sciences et Technologies de Lille, B.P. 108, 59652 Villeneuve d’Ascq Cedex, France Richard J. Chater, Stephen J. Skinner, John A. Kilner Centre for Ion Conducting Membranes (CICM), Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, UK ABSTRACT When used as ceramic membranes for the electrically driven separation of oxygen from air, BIMEVOX materials allow the production of high oxygen fluxes at moderate temperature, 300600°C. However, 18O/16O Isotope Exchange Depth Profile Technique revealed low kinetics of oxygen transfer at the surface of these ceramics when studied under equilibrium. These kinetics were considerably enhanced when a current was applied. The same membranes were characterized under working conditions using X-ray synchrotron and neutron radiations. Their dynamical transformation under bias was confirmed and explained by a slight reduction of the BIMEVOX electrolyte under working conditions. INTRODUCTION Bismuth-based materials exhibit attractive oxide ion conductivity. Among these, the BIMEVOX compounds are considered as the best oxide ion conductors at moderate temperatures, 300-600°C. They are derived from the parent compound Bi4V2O11 by partial substitution for vanadium with a metal. A wide range of elements (Cu, Co, Ni, Ta, Nb, Sb…) are able to substitute for vanadium and this allows the stabilization at room temperature of the highly conductive γ form of the parent compound [1]. BICUVOX.10, for instance, is obtained by partial substitution for vanadium with 10% of copper. Because of their high performances, these materials could be used as membrane for electrically driven Ceramic Oxygen Generators. The principle of such a device is shown in figure 1. It is very similar to the Solid Oxide Fuel Cell (SOFC) and relies on the ability of oxide ions to migrate through a ceramic material under an electric field. In a first step, oxygen molecules are dissociated into oxide ions at the cathode according to the reaction: O2 + 4e- → 2O2These oxide ions migrate, under the influence of the electric field, to the anode where they recombine into oxygen molecules according to the reverse reaction. This process allows for the production of controlled amounts of very high purity (>99.99%) oxygen, which can be delivered under pressure without the use of any mechanical device. Two steps govern the oxygen transport is such membranes i) the oxygen exchange at the surface of the ceramic, ii) the oxygen diffusion through the ceramic. The limiting step in the
Mat. Res. Soc. Symp. Proc. Vol. 756 © 2003 Materials Research Society
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whole process is often the gas-solid transfer at the surface of the material and, to help this transfer, electrode materials are usually added at the surface of the membrane.
air ecathode O2 + 4 e- →
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